1. Field of the Invention
The present invention is generally related to receivers and, more particularly to a system and method for filtering the baseband signal in a receiver.
2. Related Art
Wireless technology has undergone vast improvements in the last several years, both in terms of communication theory and hardware design. Improved hardware has sped the development of digital communications, and the digital nature of the communications link has allowed engineers to transfer larger amounts of data across a wireless link. Moreover, the improvements in hardware have allowed transmitters and receivers to operate at higher and higher frequencies, thus making even higher data rates possible.
In the push to operate at higher frequencies, the hardware has become exceedingly complex, and the signal to noise ratio (SNR) requirements have become more stringent in terms of narrow pass bands and steep selectivity for filtering devices. In making receivers comply with these requirements, development has focused on two primary methods of converting the high frequency radio signals to baseband (i.e. DC) signals, from which the communications information can be extracted. These two methods are the so-called super-heterodyne receiver, and the direct conversion receiver.
In the super-heterodyne receiver, the receiver receives the high frequency radio signal and converts it down to baseband through at least one intermediate frequency (IF). At the intermediate frequency, the receiver filters out a majority of the noise from the adjacent channels (both upper and lower). The filtering is done at intermediate frequencies because high frequency filters are expensive and generally difficult to realize because the bandwidth, as the percentage of the center frequency, is very low. Thus, lowering the high frequency signal to an intermediate frequency allows the designer to use realizable filters. Generally these filters take the form of intermediate frequency surface acoustic wave (IF SAW) filters. However, these filters are expensive and are physically large.
In contrast, the direct conversion receiver (DCR), directly converts the received high frequency radio signal to a baseband signal, rather than converting through one or more intermediate frequency steps. Thus the IF SAW filters are not required. This saves a substantial amount of cost and reduces the size of the receiver. The channel filtering is achieved by using active low pass filters. These active low pass filters can be integrated on a chip along with the other receiver elements, thus reducing cost and size. However, unlike the IF SAW filters (which are passive and hence noiseless), the active filters are quite noisy.
Further, these active low pass filters have a limited dynamic range. Dynamic range is defined as the difference between the noise floor of the filter and the maximum signal level of an adjacent channel that can be tolerated. The dynamic range requirement of the active filters determines important design parameters such as the size and current consumption of these components. A larger size chip typically equates to higher cost, and minimizing current consumption in battery powered portable units is very important in prolonging battery life. As such, one skilled in the art can see that, although direct conversion receivers are a conceptually simpler solution to the high frequency radio receiver, in practice direct conversion receivers can be more difficult to efficiently implement.
Most of the focus in the development of direct conversion receivers has centered on achieving a high dynamic range in a small size and low cost. When inexpensive, single pole filters are included in the design, the adjacent channels are suppressed by only a few decibels (dBs). For this reason, typical designs include filters that have a steep selectivity curve to allow the desired signal to pass, while attenuating the unwanted adjacent signals. However, the filters that allow this type of steep selectivity are typically very expensive. Thus, there exists a need in the industry for an effective and inexpensive filtering device, which adds very little noise to the overall system.